The use of a single two wire transmission line for determining the state of each of a plurality of bi-state devices has been proposed in the prior art. For example, U.S. Pat. Nos. 4,027,286 (Morosko), 3,550,090 (Baker) and 3,895,286 (Ono), all disclose the use of resonant circuits in conjunction with the two wire line. In addition Morosko employs phase comparison electronics in his detector circuitry for determining the state of each bi-state device. Baker and Ono, on the other hand, employ amplitude comparison electronics in their detector circuitry. However, the resonant circuits of Baker and Ono are connected in parallel with each other and the excitation source.
Notwithstanding the resonant detection techniques disclosed above, improvements are needed to enable these sensors to perform more satisfactorily under a wider variety of intended applications.